The present invention relates to a method of removing protic impurities from battery electrolytes by means of chemical adsorption.
In the lithium batteries customarily employed (secondary and primary battery cells), use is generally made of electrolytes which consist of electrolyte salts such as LiPF6, LiBF4, LiAsF6, lithium methides, lithium imides or lithium triflates and a mixture of solvents, mainly organic carbonates such as propylene carbonate, ethylene carbonate or butylene carbonate, ethers such as dimethyl ether and propionates such as methyl propionate or ethyl propionate.
These electrolyte solutions normally comprise, despite the high purity of the individual components, protic impurities such as water, alcohols, peroxides. However, the electrolyte salts in the electrolyte solutions are extremely sensitive to these impurities and decompose, for example, to HF, LiF, POF3 or PxOyFz, and to various oxofluorophosphoric acids (RaPbOcFd).
These decomposition products are very damaging to the battery cells since they attack the cell components, i.e. cathode and anode, and have a tremendous influence on the covering layer formation on the electrodes. This significantly shortens the life of a battery.
Since HF in particular is very aggressive in this respect, it is necessary to reduce the HF content of the electrolyte mixtures, which is normally 50-80 ppm, substantially. For most applications, an HF content of less than 30 ppm is desired.
The water content of the electrolyte mixture should, if possible, likewise be very low so that these decomposition products cannot occur at all to the previous extent. A very low water content (e.g. less than 20 ppm) is therefore desirable. The methods previously employed for reducing the water content in conventional ways are not effective enough.
The Canadian patent application 2,193,119 describes a method in which the acidic impurities are separated off by means of hydrogen-free chlorides, bromides or iodides. However, this method is also not optimal, since the reaction products formed, viz. HCl, HBr and HI, are still present in the electrolyte mixtures during and to a small extent even after the removal by distillation and can therefore trigger further reactions.
Furthermore, it has been proposed in the literature (J. Electrochem. Soc. Vol. 143, No. 12, 3809-3819, 1996) that HF be neutralized using a base such as tributylamine. However, the reaction products of HF with tributylamine remain in the electrolyte, which is a great disadvantage. It has been confirmed that this reduces the cycling efficiency.
It is therefore an object of the present invention to find a method of removing protic impurities, in particular water or HF, which is simple, quick and effective to carry out and which can reduce the content of water and HF to less than 30 ppm.
It has now been found that fixing the basic group on a support enables the disadvantages described to be avoided and makes it possible to carry out the removal of the protic impurities from battery solvents simply and very effectively by means of chemical adsorption.
The invention accordingly provides a method of purifying electrolyte solutions for lithium cells, characterized by the following steps:
a) adding a base fixed on a support material (adsorbent) which chemically adsorbs the protic impurities, and
b) separating off the adsorbent.
The electrolyte solutions according to the invention consist essentially of electrolyte salts such as LiPF6, LiBF4, LiAsF6, lithium methides, lithium imides or lithium triflates, preferably from 0.7 to 1.8 mol/l, and solvent mixtures selected from among the organic solvents organic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate and further organic carbonates and propionates such as methyl propionate or ethyl propionate, formates such as ethyl formate or methyl formate, acetates such as methyl acetate, ethyl acetate, halogenated carbonates such as chlorinated ethylene carbonate, fluorinated ethylene carbonate, fluorinated propylene carbonate or fluorinated ethylene carbonate and also ethers such as dimethoxyethane.
Suitable adsorbents are generally all those which comprise basic groups bound to a support and can thds chemically adsorb and thds neutralize the protic impurities.
Basic groups used are preferably amino groups, in particular primary amino groups. As support, preference is given to using silicon dioxide, polystyrene or other plastics.
In particularly preferred embodiments of the present invention, the basic groups selected are, for example, xe2x80x94NH2 groups which are bound to a silicon dioxide matrix via propylene groups. This product is commercially available as Lichroprepo(copyright) xe2x80x94NH2.
Preference is also given to using Amberlite resins having xe2x80x94NH2 groups fixed thereto.
The purification according to the invention can be carried out in various ways.
The first possibility is to finish mixing the electrolyte solution and then to add the adsorbent for separating off the protic impurities and subsequently separating it off again.
In the second possibility, the solvents required for the electrolyte solution are first mixed and the adsorbent is then added. After adsorption is complete, the adsorbent is separated off again and only then is the electrolyte salt mixed in.
The adsorbent can, as one alternative, be introduced into the respective mixture while stirring and subsequently be separated off again by filtration. The reaction time can be chosen freely, but it is preferably kept as short as possible; on the bass of experience, brief stirring for up to 10 minutes is sufficient for the adsorption to be complete.
Another alternative is to introduce the adsorbent into a column. The solution to be purified is, as is customary, passed through the column of adsorbent by means of a pump.
The adsorbent has to be free of water; it is preferably dried well before use. Preferably, it is dried at about 100xc2x0 C. under reduced pressure for a few days, cooled and subsequently stored with exclusion of moisture or, better, used immediately.
Preference is given to adding from 0.2 to 3% by weight of adsorbent to the electrolyte solutions to be purified. Particular preference is given to an adsorbent content of from 0.4 to 1% by weight.
The adsorbent is separated off by filtration or the like. These conventional methods are known to those skilled in the art.
This gives purified electrolyte solutions which meet the demanding requirements for a low water and HF content. The battery solvents purified according to the invention have values for the water and HF content of less than 20 ppm.
The electrolyte solutions of the invention therefore display improved properties such as higher cycling efficiency and longer life when used in lithium-ion and lithium batteries.
The invention thds also provides electrolyte solutions which are suitable for lithium cells (primary or secondary), which are characterized in that they have been purified by the method described here.
It is assumed that a person skilled in the art can utilize the above description in its fullest scope even without provision of further details. The preferred embodiments are therefore to be interpreted only as a descriptive and in no way limiting disclosure.
The complete disclosure of all applications, patents and publications mentioned above and below are incorporated by reference into the present application.